This application claims priority under 35 U.S.C. xc2xa7119 and/or 365 to 0001619-6 filed in Sweden on May 3, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to optical transmission techniques, particularly single mode integrated optics, for tele and data communication. Specifically, the invention relates to an apparatus and a method for wavelength selective switching of optical wavelength channels.
In order to increase the capacity of an optical transmission line (point to point connection) or in an optical network (multipoint to multipoint connection) there are today a number of known techniques. One of these comprises to utilize wavelength division multiplexing (WDM) of transmission channels to enhance the utilization of bandwidth in the network, which, however, requires access to apparatuses capable of switching, multiplexing and demultiplexing transmission channels, which are transmitted on different so-called optical carrier wavelengths in the network. In order to reconfigure an optical network wavelength selectively, tunable wavelength selective switches are required.
Through the publication Theoretical Investigation of a Wavelength Selective Switch Architecture Based on a Bragg Grating-Assisted MMIMI Configuration, IEEE Photonics Techn. Lett., Vol. 11, No. 7, July 1999, pages 839-841, is known an architecture for multichannel wavelength selective switching based on an MMI-based Michelson interferometer structure coupled in parallel provided with Bragg gratings (MMI, Multi Mode Interference).
The switching structure comprises a number of Michelson arms connected to an MMI structure, where each Michelson arm comprises an MMIMZI structure (MMIMZI, Multi Mode Interference Mach-Zehnder Interferometer) and a plurality of phase control units operating in reflection mode and connected in parallel to said MMIMZI structure. Each phase control unit comprises a plurality of serially coupled phase control elements and Bragg gratings and is arranged for phase control of a respective sub-group of the total number of channels, which are handled by the structure.
A drawback of the above-mentioned Michelson-based wavelength selective switch architecture is that problems with channel crosstalk may occur, e.g. due to process-dependent variation effects, despite the fact that the theory indicates low crosstalk. Further, the switch is assumed to be particularly sensitive for scattering losses, since it is based on reflection in long Michelson arms.
If many channels shall be handled, apparatuses, which are relatively complicated and relatively difficult and costly to realize, are required.
It is an object of the present invention to provide an apparatus and a method for completely individual wavelength selective switching of an optical wavelength multiplexed signal comprising a plurality of optical wavelength channels, which exhibit enhanced performance.
It is a further object of the invention to provide such an apparatus and such a method for wavelength selective switching, which can exhibit low channel crosstalk.
It is a further object of the invention to provide an apparatus and a method for completely individual wavelength selective switching, which may constitute an alternative to known techniques.
It is a particular object of the invention to provide an apparatus and a method for completely individual wavelength selective switching of many channels, wherein some channels can be individually switchable and other channels can belong to channel groups, the channels in each channel group being switched together.
It is a further object of the invention to provide an apparatus and a method for wavelength selective switching, wherein the ratio of the dimension of the switching (the sum of the number of inputs and outputs) and the maximum dimension required of the MMI couplers utilized during switching are large.
It is a further object of the invention to provide an apparatus and a method for wavelength selective switching, which, during use, will exhibit low power losses and are insensitive to instabilities.
Further objects of the present invention will be apparent from the detailed description below.
The above-mentioned objects are according to a first aspect of the present invention attained by an apparatus for wavelength selective switching of a plurality of optical wavelength channels, which comprises two MMI waveguides interconnected by at least two Mach-Zehnder waveguide structures arranged in parallel, of which each is arranged to transmit a respective portion of the intensity of said plurality of optical wavelength channels. According to the invention each Mach-Zehnder waveguide structure comprises a demultiplexing unit, a multiplexing unit and at least two waveguides arranged in parallel, wherein different channels are handled in parallel in different ones of the waveguides arranged in parallel.
The demultiplexing unit is more specifically arranged for demultiplexing of said plurality of optical wavelength channels into at least two channel groups, each waveguide is arranged in parallel for transmission of a respective of said channel groups to the multiplexing unit and is further provided with a respective multichannel wavelength selective phase control unit arranged for individual phase control of at least some channels in the respective of said channel groups, which is transmitted to the multiplexing unit, and the multiplexing unit is arranged for multiplexing of said channel groups.
According to a first embodiment, the demultiplexing unit and the multiplexing unit are each comprised of an MMIMZI-based device, where each MZI arm comprises a phase control element. These MMIMZI-based devices are preferably connected by the two waveguides arranged in parallel.
Preferably, each multichannel wavelength selective phase control unit is comprised of an MMIMZI-based device (MI, Michelson), where each MI arm at least comprises, as seen from the MMI waveguide, a first phase control element, a first Bragg grating, a second phase control element and a second Bragg grating, wherein the first Bragg grating is arranged for reflection of at least a first channel in the respective channel group, which is handled by the phase control unit, the second Bragg grating is arranged for reflection of at least a second channel in the respective channel group, which is handled by the phase control unit, and the phase control elements are arranged for phase control of the respective channels, which are transmitted through them.
In a second embodiment, the demultiplexing unit and the multiplexing unit are together comprised of an MMIMI-based configuration, where each MI arm comprises a respective MZI based demultiplexing/multiplexing unit and is arranged to transmit a respective portion of the intensity of said plurality of optical wavelength channels. In this respect, two waveguides are arranged in parallel at each demultiplexing/multiplexing unit, where each waveguide is provided with a respective multichannel wavelength selective phase control unit arranged for individual phase control of at least some channels in the respective channel group, which is transmitted in the waveguide.
Preferably, each of the multichannel wavelength selective phase control units comprises, as seen from the demultiplexing/multiplexing unit, a first phase control element, a first Bragg grating, a second phase control unit and a second Bragg grating, wherein the first Bragg grating is arranged for reflection of at least a first channel in the respective channel group, which is handled by the phase control unit, the second Bragg grating is arranged for reflection of at least a second channel in the respective channel group, which is handled by the phase control unit, and the phase control elements are arranged for phase control of the respective channels, which are transmitted through them.
This embodiment is further characterized therein that each MI arm in the MMIMI-based configuration comprises a Bragg grating localized between the MMI waveguides of the MMIMI-based configuration and said MZI-based demultiplexing/multiplexing unit, the Bragg grating being arranged for reflection of at least some of said plurality of optical wavelength channels, and therein that at least some MI arm in the MMIMI-based configuration comprises a phase control element localized between the MMI waveguides of the MMIMI-based configuration and said Bragg grating, which is arranged for reflection of said at least some of said plurality of optical wavelength channels.
The apparatus according to the invention may be realized as an Nxc3x97M switch and in the detailed description below particularly 1xc3x972, 4xc3x974 and 2xc3x972 switches will be described.
The above-mentioned objects are according to a second aspect of the present invention attained by a method for wavelength selective switching of a plurality of optical wavelength channels, in an apparatus comprising two MMI waveguides, interconnected by at least two Mach-Zehnder waveguide structures arranged in parallel, of which each is arranged to transmit a respective portion of the intensity of said plurality of optical wavelength channels.
The method comprises that different channels are processed in parallel in each Mach-Zehnder waveguide structure.
More specifically, the steps of demultiplexing said plurality of optical wavelength channels into at least two channel groups by means of a demultiplexing unit, transmitting the respective channel group to a multiplexing unit by means of a respective waveguide connected in parallel between the demultiplexing unit and the multiplexing unit, are performed. At least some of the channels in the respective channel group, which is transmitted to the multiplexing unit, are individually phase-controlled by means of a respective multichannel wavelength selective phase control unit arranged at the respective waveguide arranged in parallel and said channel groups are multiplexed by means of the multiplexing unit.
An advantage of the present invention is that the switching capacity is considerably increased when different channels are processed in parallel.